Chopped glass fiber strands are commonly used as reinforcement materials in thermoplastic articles. Typically, the glass fibers are formed by drawing molten glass into filaments through a bushing or orifice plate, applying a sizing composition containing lubricants, coupling agents and film-forming binder resins to the filaments, gathering the filaments into strands, chopping the glass fiber strands into segments of the desired length, and drying the sizing composition. These chopped fiber strand segments are thereafter mixed with a polymeric resin, and the mixture supplied to a compression- or injection-molding machine to be formed into glass fiber reinforced plastic articles. Typically, the chopped fiber strands are mixed with pellets of a thermoplastic polymer resin, and the mixture supplied to an extruder wherein the resin is melted, the integrity of the glass fiber strands is destroyed, the fiber strands are dispersed throughout the molten resin, and the fiber strand/resin dispersion is formed into pellets. These pellets are then fed to the molding machine and formed into molded composite articles having a substantially homogeneous dispersion of the glass fiber strands throughout.
Unfortunately, however, chopped glass fiber strands made via such processes are typically bulky and do not flow well. Consequently, such chopped strands are difficult to handle and have been problematic in automated processing equipment.
One attempt at solving this problem has been to compact the chopped fiber strands into denser rod-shaped bundles or pellets to improve their flowability, and to enable the use of automated equipment to weigh and transport these pellets for mixing with the thermoplastic polymer resins. Such a process is disclosed in U.S. Pat. No. 4,840,755, wherein wet chopped fiber strands are rolled, preferably on a vibrating carrier, to round the strands and compact them into denser, cylindrically shaped pellets. While such methods and apparatus tend to provide denser, more cylindrically shaped pellets exhibiting better flowability, they are undesirably limited in certain respects. For example, in such pellet-forming processes, the pellet size and fiber content are generally limited by the size and number of fibers in the chopped strand, because the process is designed to avoid multiple chopped strand segments from adhering together to form pellets containing more fibers than are present in a single chopped strand. Consequently, to obtain pellets having a suitable bulk density and a sufficient ratio of diameter to length to exhibit good flowability, the fiber strand from which the segments are chopped usually must be formed of a large number of filaments. However, increasing the number of filaments required to be formed and combined into a single strand undesirably complicates the forming operation.
In an attempt to overcome these shortcomings, U.S. Pat. No. 5,578,535, which is herein incorporated by reference, discloses glass fiber pellets that are from about 20 to 30 percent denser than the individual glass strands from which they are made, and from about 5 to 15 times larger in diameter. These pellets are prepared by (i) hydrating cut fiber strand segments to a level sufficient to prevent separation of the fiber strand segments into individual filaments but insufficient to cause the fiber strand segments to agglomerate into a clump; and (ii) mixing the hydrated strand segments by a suitable method for a time sufficient to form pellets. Suitable mixing methods include processes that keep the fibers moving over and around one another, such as tumbling, agitating, blending, commingling, stirring and intermingling. Although these pellets can be made by such diverse mixing methods, it has been discovered that many of these methods are either too inefficient to be used commercially, or cannot be adequately controlled to produce sufficiently uniform pellets to provide the composite article made therefrom with the strength characteristics comparable to a composite article made from non-pelleted chopped fiber strands. For example, the use of a modified disk pelletizer frequently results in excessive residence time of the formed pellets within the mixer, which causes the pellets to rub against each other for an excessive period, which in turn results in degradation of the pellets, due to their abrasive nature. Such pellet degradation ultimately reduces the strength characteristics of the molded composite articles.
Another problem commonly known to pellets made from fiber strands that are made for use as reinforcements in composites and other fiber-reinforced products is discoloration. This discoloration is typically seen as an undesirable yellowing of the pellets that is thought to be related to some of the materials used to size the fiber strands, including, but not limited to, the binders and film formers used in the sizing compositions used to treat the fiber strands. Such discoloration of the pellets tends to cause a discoloration of the composite article made therefrom.
Accordingly, a need exists for an efficient pellet-forming process that controllably yields a uniform glass fiber pellet product that provides strength characteristics equal to non-pelleted chopped strand fibers, when such pellets are used to make molded composite articles. Further, a need exists for a sizing composition for use in preparing chopped strands and glass fiber pellets which have little or no coloration, and no effect on the color of the molded composite articles prepared from them. Such needs are fulfilled by the various embodiments of the present invention, which is summarized and described in detail below.